So I’ve mentioned before that I run an NTP server. Stratum 2, which means it gets its time from a “Stratum 1,” which is set directly to something reliable. The main goal of NTP is to keep clocks in sync, and it’s pretty accurate, down to a fraction of a second, which is more accuracy than most people need. All of my computers will now agree on the time down to a second.
The ultimate source, of course, is the atomic clock. But there isn’t an atomic clock, per se. There’s actually an array of them, each using cesium or hydrogen as an atomic reference. Collectively they form “the” atomic clock, which is used as a frequency standard.
It’s all well and good to keep your computer clock (and wristwatch, and microwave, and oven, and wall clock…) synced within a second, but some things need more accuracy. The USNO (US Naval Observatory, in charge of maintaining the atomic clock system) explains one common scenario well: systems for determining one’s location, such as GPS and LORAN “are based on the travel time of the electromagnetic signals: an accuracy of 10 nanoseconds (10 one-billionths of a second) corresponds to a position accuracy of 10 feet.” There are also lots of other scientific uses for extremely precise time, many of which I couldn’t even begin to understand the basic premise of. But suffice it to say that there are actually a lot of times when knowing the time down to the nanosecond is important.
Things like NTP don’t cut it here. You can get down to the millisecond, but you need to be about a million times more accurate. (A millisecond is a thousand microseconds, which is a thousand nanoseconds.) So how do you keep the exact time? It turns out that there are actually several ways. One way (decreasingly common) was to keep an atomic clock of your own. You can buy a “small” (the size of a computer…ish) device that has cesium or hydrogen or rubidium inside of it, which keeps pretty accurate time. Over time it’ll wander, but at least short-term, it’s quite accurate.
One of the first ways is WWV, a shortwave radio station. (And it’s Hawaiian sister station, WWVH.) They run continuously, disseminating the exact time via radio as observed from the atomic clock system. In the past I’ve synced my watch to this source. More notable, in a behind-the-scenes type of way, is WWVB, a low-frequency (60 kHz) radio broadcast. This is what all your “atomic wall clocks” sync to. (Incidentally, I’ve read that most of them are fairly cheaply built, meaning that their time is really not accurate to more than a second.) Another interesting sidenote is the deal with their antennas: a quarter-wavelength antenna at such a low frequency is 1,250 meters tall, or about 4,100 feet (nearly a mile). But with some wacky designs they can overcome this (although pouring 50,000 Watts into it also helps).
The problem with “straight” receivers for WWVB, though, is that you have to figure in the time it takes for the signal to reach you, which is rarely done all that well (if at all). Instead, a more common technology is used: GPS.
It turns out that GPS carries insanely accurate time. Wikipedia has a really good article on it. Each GPS satellite carries an atomic clock onboard, and people on the ground keep it synced (with nanosecond accuracy) to the atomic system. There’s some funky correction going on to keep things perfectly accurate. GPS has a claimed accuracy of 100 nanoseconds, although people have found that it’s actually about ten times better, down to 10 nanoseconds or so.
As an aside, GPS in general is an interesting read. There’s a lot more going on than meets the eye. I recently dug up an old GPS and wondered if it needed an “update” to get new satellite positions: with ham satellites, we get periodic updates for our tracking software to account for changes in their path. GPS has a neat solution, though: the satellites broadcast this data. Actually, more accurately, they broadcast all the data for all the satellites, so that seeing one satellite will fill you in on the whole setup. There used to be Selective Availability, basically a deliberate introduction of error into the signal. The premise was that we didn’t want enemy forces using it: imagine a GPS-guided rocket, for example. So we introduced error of about 30 meters for a while. Ironically, it was ended because our own troops (before Iraq) couldn’t get the military units, so they were just buying off-the-shelf civilian units and incurring the decreased accuracy. So Selective Availability has been turned off, and there are indications that it was permanent. A third interesting tidbit is that the GPS satellites carry much more than might meet the eye, including equipment monitoring for nuclear detonations.
The timekeeping problem is what to do when you get the time at the GPS, though. High-end GPS units will provide a pulse-per-second signal, which you cna hook up to a computer via serial, and achieve great accuracy. But there are all sorts of considerations I never thought of. Between the time it actually charges the pin and the time the operating system has processed it takes a little bit of time, os there are special kernel modifications available for Linux and BSD to basically get the kernel directly monitoring the serial port, to greatly speed up its processing. I also discovered the Precision Time Protocol (commonly known by its technical name, IEEE 1588), which is designed to keep extremely accurate time over Ethernet, but apparently requires special NICs to truly work well.
I’ve also learned another interesting tidbit of information. CDMA (which is a general standard, not just the cell phone technology that Verizon uses) apparently requires time down to the microsecond to keep everything in sync, such as your multiple towers and all the units (e.g., phones) in sync and transmitting at the right times. So the easiest way to keep all of their towers in sync to a common standard was to put a GPS receiver at each tower and sync the system to that. Thus CDMA carries extremely accurate time derived from GPS, which has led to some interesting uses. It’s hard to get a GPS signal indoors, so they now make CDMA time units–they sit on a CDMA network in receive-only mode, getting the time but never taking the “next step” of actually affiliating with the network. This lets people get GPS-level accuracy inside buildings.
